Liquefied petroleum gas (LPG), primarily comprised of propane, may be used to fuel an internal combustion engine. LPG may be delivered to an engine in various phases (e.g., liquid and gaseous). In some examples, engine systems may only include direct injectors; however, direct injection of LPG may not be suitable under relatively hot conditions because liquid LPG may vaporize. In other examples, engine systems may only include port injectors; however, port injection of LPG may not be suitable under relatively cool conditions because gaseous LPG may condense. To address this issue, some engine systems may include both direct and port injection in what is commonly referred to as port fuel direct injection (PFDI). In such PFDI systems, fuel is lifted from a fuel tank by a lift pump and is then either directly supplied to the port injection fuel rail, or is supplied to a higher pressure direct injection pump before continuing on to the direct injection fuel rail.
However, the inventors herein have recognized potential issues with such PFDI systems when fueling with LPG. As one example, liquefied petroleum gas (LPG), has a relatively low super critical temperature of about 96° C. If the temperature of the direct injection rail increases above the super critical temperature of the LPG, LPG in the direct injection rail may vaporize before it is injected to the engine which may result in air-fuel ratio errors and an increase in engine knock.
As another example, PFDI systems typically include fuel return lines that return excess fuel to the fuel tank. For example, a fuel return line may couple the direct injection fuel rail to the fuel tank for returning fuel from the direct injection fuel rail to the fuel tank. However, such return lines increase the temperature of the LPG in the fuel tank, and thereby increase vaporization of the LPG in the fuel system. If LPG vapors reach an inlet of the direct injection pump, they may impair and/or reduce the volumetric efficiency of the pump. As such, direct injection pumps may require a pump cooling system and/or significant pressure enhancement, both of which increase costs and complexity of the fuel system.
In one example, the issues described above may be at least partially addressed by a method comprising: supplying liquefied petroleum gas (LPG) from a fuel rail to a direct injection injector that injects fuel directly into a cylinder of an engine, and flowing LPG from the fuel rail to an intake fuel injector that is not a direct injection injector without returning the LPG to a fuel tank, where the intake fuel injector injects fuel, from a position outside the cylinder, into an intake passage that feeds the cylinder.
In another representation a method for an engine may comprise pumping: pumping LPG from a fuel tank to a direct injection (DI) fuel rail; directly injecting liquid LPG into at least one cylinder of the engine via one or more direct injectors coupled to the DI fuel rail; and ejecting vaporized LPG from the DI fuel rail to an intake injector that is not a direct injector without returning the vaporized LPG to the fuel tank. The method may further comprise deactivating a direct injection or higher pressure pump.
In another representation a fuel system may comprise: a lift pump; a direct injection fuel rail coupled to the lift pump via a first fuel supply line; a port injection fuel rail coupled downstream, and in series with, the direct injection fuel rail via a second fuel supply line, where the port injection fuel rail and second fuel supply line are at a lower pressure than the direct injection fuel rail; an injector coupled between the direct injection fuel rail and the port injection fuel rail to supply fuel from the direct injection fuel rail to the port injection fuel rail; and a controller in electrical communication with the lift pump and injector, the controller including computer-readable instruction stored in non-transitory memory to: feedback control the lift pump in a low pressure direct injection mode; and feedback control the injector to maintain a pressure of the port injection fuel rail below a pressure of the direct injection fuel rail and below an LPG liquid-to-gas phase change pressure, the LPG liquid-to-gas phase change pressure based on a temperature of the port injection fuel rail.
By low pressure direct injecting LPG into the engine cylinders, the higher pressure direct injection pump may be removed/omitted from the fuel system, reducing the cost and complexity of the fuel system. In examples where the direct injection pump is retained in the fuel system, but is rendered inactive, pump longevity may be enhanced by reducing the amount of fuel vapors entering the pump inlet. Fuel vapors may be reduced by omitting fuel return lines in the fuel system, and cooling the direct injection rail with vapors sourced from the direct injection rail that have been depressurized (and thus cooled). Omitting the fuel return lines further reduces the cost and complexity of the fuel system.
Further, by cooling the direct injection rail with the depressurized LPG vapors, vaporization of LPG in the direct injection rail may be reduced, thereby reducing air-fuel ratio errors and engine knock when direct injecting LPG. Correspondingly, by heating the cooler, depressurized LPG vapors contained between the direct injection fuel rail and the port injection fuel rail, with the hotter direct injection fuel rail, condensation of LPG in the port fuel injection rail may be reduced, thereby reducing air-fuel ratio errors and increasing engine performance and robustness. As a result, robust LPG engine fueling may be achieved over a wider range of ambient and engine operating temperatures.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.